1. Field of the Invention
The invention relates to a method for purifying carboxylic acids from fermentation broths, and also a device for carrying out the method according to the invention. The isolation of carboxylic acids, which cannot be separated by distillation or only with difficulty, proves to be very complex.
2. Description of the Related Art
The decisive factor for the industrial use of carboxylic acids, which are generated by fermentation of carbohydrate-containing substrates by various microorganisms, is the economic viability and efficiency of the separation and purification of the lactic acid from these aqueous fermentation solutions, which also comprise, in addition to the carboxylic acid or the carboxylic acid salts, further organic acids, other by-products of the fermentation, microorganisms and constituents thereof and also residues of the substrates such as sugar. These impurities impair the subsequent processing of the carboxylic acids generated. For example, lactic acid is polymerized to polylactic acid in order to produce biologically degradable plastics. In this case, an extremely pure monomer must be used in order to attain a high degree of polymerization of the lactic acid. This has long been known and appears, for example, in J. Dahlmann et al, British Polymer Journal, Vol. 23 (1990), pp 235-240.
The same is known for succinic acid for example. The qualities of the succinic acid produced may be differentiated by subdivision into a technical grade having a succinic acid content of at least 97% by mass and a succinic acid specifically suitable for use for polymerization (polymer grade) having a content of at least 99.5% by mass.
A number of patents describe the production of succinic acid from fermentation solutions, including:                extractive processes using extraction agents such as tributylamines, trialkylamines, olefins, various alcohols and aromatic hydrocarbons;        processes using calcium hydroxide and sulfuric acid, in which gypsum occurs as by-product;        processes using electrodialysis;        thermal methods such as fractional distillation or thermal stepped chromatography;        high pressure extraction using CO2; and        membrane methods such as reverse osmosis and other filtration processes;wherein coupling of these processes and supplementation by additional steps corresponding to the prior art are also discussed. Such methods are described, inter alia, in patent specifications DE 69821951 T2; DE 69015233 T2; DE 69015019 T2; DE 69006555 T2; DE 69015019; DE19939630C2; DE 60028958T2; and DE 10 2004 026152 A1.        
Numerous methods relating to the purification of lactic acid are also known. For example, it is taught in a number of patents to use distillation for purifying lactic acid from aqueous solutions. Such a method is made use of in EP 0986532 B2. DE 10 2007 045 701 B3 discloses a combined extraction with linear n-trioctylamine (TOA) and distillation. Other possibilities known from the literature are electrodialysis or esterification with an alcohol after which distillation and then hydrolysis of the ester formed are likewise carried out. These methods are extremely cost-intensive. Distillation also has the disadvantage that a part of the carbohydrates are also always co-extracted which leads to a deterioration in the yield of the overall process and makes it difficult to isolate the product.
Processes using calcium hydroxide and sulfuric acid are also known in which gypsum occurs as by-product in large amounts. In this context, it was also found that lactic acid can be isolated by chromatographic methods from, for example, a fermentation broth acidified with sulfuric acid, which still comprises ammonium and sulfate ions in addition to free lactic acid. By way of example, DE 69815369 T2 describes, inter alia, the removal of lactic acid from aqueous mixtures by adsorption on a solid adsorbent, a solid adsorbent preferably being used here which adsorbs lactic acid as opposed to lactate. In particular, it is possible to use weak anion exchangers for the isolation of lactic acid according to the above documents. DE 10 2009 019 248 A1 further describes chromatographic methods for purifying organic acids, especially lactic acid, by carrying out simulated moving bed chromatography.
WO 2006/124633 A1 describes a process for preparing ammonium lactate by fermentation. The ammonium salt of lactic acid is formed in the fermentation which may be separated from the fermentation solution, for example, by extraction. The ammonium salt can be very readily cleaved in a subsequent step using weak acids or carbon dioxide. The free lactic acid is thus obtained which can then be purified, for example, by distillation.
WO99/19290 describes a lactic acid fermentation with subsequent filtration and extraction, where the extraction can be an adsorption. In this case, the type of interaction in the solid phase adsorption is not disclosed. A similar method is disclosed in WO93/06226, in which the adsorption solid phase is equipped with tertiary amino groups and the rate of production of free acid is thereby increased. EP0135728 also teaches the isolation of enzymatically generated carboxylic acids via adsorbents which are equipped with tertiary amino groups. The fermentation is carried out in this case by cells immobilized on columns.
DE102010025167A1 discloses a method for separating, recovering and purifying succinic acid. Separation of the biomass from the fermentation broth in this case is achieved in two successive steps. The dicarboxylic acid solution is then separated from the biomass-free fermentation broth by simulated moving bed chromatography, followed by ultra-purification and a multi-stage evaporation and crystallization. A disadvantage of this method is the high energy expenditure caused by the multi-stage evaporation of the dicarboxylic acid solution.
WO2011082378A2 teaches the purification of succinic acid from a fermentation broth comprising ammonium succinate. Ion exchange columns are used to separate the ammonium succinate from the fermentation broth and to generate succinic acid. In this case, ammonium sulfate is obtained in the raffinate and succinic acid in the extract. The raffinate is subjected to a crystallization, for which purpose it is concentrated. The concentration of the raffinate, i.e. of the ammonium sulfate, is carried out by reverse osmosis and/or evaporation. This step is generally known. The workup of the ammonium sulfate is essential for the economic viability of the preparation of succinic acid by fermentation, since in the conversion of ammonium succinate to succinic acid from the fermentation broth, about one and a half times the amount of ammonium sulfate is obtained.
A disadvantage of many processes, therefore, is that the practical implementation of the methods is associated with considerable technical and energy expenditure.